Electrical stimulation of biological tissues is currently used or under active consideration for use in the treatment of a variety of clinical problems. While the possibility of selective electrical stimulation of a localized region within the body by completely non- invasive means has been postulated on theoretical grounds, such stimulation has yet to be demonstrated experimentally. However, the selective stimulation of a localized region within the body can be achieved by implanting a very small diode at the desired site and then applying a pulsed radio-frequency field by means of electrodes or a coil on or near the surface of the body. We have derived reasonably complete theoretical descriptions of the two systems on the basis of an idealized model in which circular electrodes or the coil are placed on the surface of a semi-infinite homogeneous medium. In our descriptions, the direct current component of diode current is related to the power input to the electrode pair or to the coil and to the geometry of the system. We are currently evaluating the two systems for the cardiac pacing of dogs and intend to extend our study to cover other stimulation applications. We also intend to derive input power-diode current relationships for more complex models.